1. Field of the Invention
The present invention concerns a method of determining conditions for an anti-reflective layer, a method of forming an anti-reflective layer and a method of forming a resist pattern by using a novel anti-reflective layer. In particular, it relates to a method of determining a condition of an anti-reflective layer for defining optical conditions such as a thickness and a refractive index condition, for example, reflection refractive index and absorption refractive index of an anti-reflective layer upon forming a resist pattern by exposing a photoresist on an anti-reflective layer formed on an underlying material by a monochromatic light, a method of forming the anti-reflective layer utilizing the above-mentioned method and a method of forming a resist pattern by using a novel anti-reflective layer. The present invention can be utilized suitably, for example, either in a case of setting an anti-reflective layer disposed for preventing the standing wave effect when photolithography is used or in a case of forming a resist pattern by using an anti-reflective film when photolithography is used upon manufacturing, for example, electronic materials (such as semiconductor devices).
2. Description of the Prior Art
For instance, in the photolithography, a KrF excimer laser beam (.lambda.=248 nm) is used and a lens of about 0.37 to 0.42 NA is mounted in most advanced steppers (projection exposing machine) at present (for example, Nikon NSR 1505 EX1, Canon FPA 4500). By using the steppers, research and development have been studied for design rule devices in a sub-half micron (0.5 um) region.
In the stepper, a monochromatic light is used as an exposing optical source. In a case of exposure by the monochromatic light, it has been generally known that a phenomenon referred to as a standing wave effect occurs. The standing wave is caused by occurrence of light interference in a resist film. That is, it is caused by interference between an incident light P and a reflection light R from the interface between a resist PR and a substrate S in the film of the resist PR.
As a result, as shown in FIG. 19, the amount of light absorbed in the resist (ordinate in the graph) fluctuates depending on the thickness of the resist film (abscissa in the graph). In the present specification, the amount of light absorbed in the resist means an amount of light absorbed in the resist itself excluding the amount of light due to surface reflection, absorption by a metal if it is present in the resist, or light outgoing from the resist. The amount of the absorbed light constitutes an energy for causing light reaction to the resist.
As can be seen from the comparison between FIG. 20 and FIG. 21, the extent of the fluctuation for the amount of the absorbed light differs also depending on the kind of underlying substrates. In FIGS. 19, 20 and 21, XP 8843 (manufactured by Shipley Co.) is used in each of the cases and Si, Al-Si and W-Si are used as the underlying material in respective cases. That is, fluctuation for the amount of the absorbed light is determined by a complex swing reflectivity (R) (in which (R) represents that it is a vector amount having a real part and an imaginary part) considering multiple interference determined by optical constants (n, k) of the underlying material (substrate) and optical constants (n, k) of the resist.
Further, in an actual device, as schematically shown in FIG. 22, unevenness is always present on the surface of a substrates. For instance, protrusions In such as poly-Si are present. Therefore, when the resist PR is coated, the thickness of the resist film varies between upper and lower portions of the step. That is, the thickness dPR2 of the resist film on the protrusion In is smaller than the thickness dPR1 of the resist film in other portions than the above. As has been described previously, the standing wave effect differs depending on the thickness of the resist film and, accordingly, fluctuation for the amount of the light absorbed in the resist changes respectively undergoing the effect of the standing wave effect. As a result, the dimension of the resist pattern obtained after exposure and development differs between the upper and the lower portions of the step.
Influence of the standing wave effect on the dimension of the pattern becomes more remarkable as the pattern is finer in a case of using a stepper of an identical wavelength and an identical number of aperture. FIGS. 23-25 show the influence of the standing wave effect on every pattern dimension in a case of using Nikon NSR 1505 EX1 as a stepper (exposure light used: .lambda.=248 nm, KrF excimer, NA=0.42) and using XP 8843 as a resist (chemically amplified type resist, a polyvinylphenol type resist containing optical acid generating agent, manufactured by Shipley Microelectronics Co.). It is apparent that the standing wave effect becomes remarkable as the pattern becomes finer (refer also to the scattering of critical dimension shift at 0.5 um, 0.4 um and 0.35 um line-and-space patterns shown by "open circles" in the drawings).
The above-mentioned trend is a phenomenon observed in common with all of resists.
The dimensional accuracy of a resist pattern in a photolithographic step upon manufacturing a device such as a semiconductor device is generally .+-.5%. Although it is considered that an accuracy coarser than .+-.5% in total may be practically tolerable. However, it is desirable that the pattern accuracy upon resist exposure is within .+-.5%, if occurrence of scattering due to other factors such as focus is also taken into consideration. For attaining the dimensional accuracy of .+-.5%, it is essential to reduce the standing wave effect.
FIG. 26 shows a dimensional variation of the resist pattern relative to the fluctuation (ordinate) for the amount of absorbed light in the resist film (abscissa). As can be seen from FIG. 26, fluctuation for the amount of absorbed light in the resist film has to be within a range of less than 6% in order to manufacture, for example, a rule device of 0.35 um.
For satisfying the above-mentioned requirement, earnest studies have been made on the anti-reflective technique in each of the fields. However, although the type of material for the underlying material and the resist to be used are known, it is not always easy to determine as to what are the conditions for the anti-reflective layer that can attain an anti-reflective effect suitable to such a case.
For instance, In the formation of a pattern on a gate structure (for example, on a W-Si film) for which an anti-reflective layer is considered indispensable, it has not yet been determined what are the condition for the anti-reflective layer that will reduce the fluctuation for the amount of the absorbed light in the resist film, for example to a range of less than 6%. Naturally, no effective anti-reflective layer material to be used for such W-Si has yet been found.
For the structure using the W-Si material as the gate, a pattern has now been formed at present, for example, by means of a multi-layer resist method or dye-incorporated resist. Accordingly, it is considered essential to establish anti-reflective technique on W-Si as soon as possible.
In such a case, if there is a means capable of determining comprehensive conditions and concrete conditions regarding an anti-reflective layer for forming a stable fine pattern on an optional underlying material (substrate) using an exposure optical source of an optional monochromatic light it can be found for the condition of the anti-reflective layer to be formed, for example, on W-Si. However, no such means has yet been proposed.